1. Field of the Invention
This invention relates to waveguide phase shifting, and more particularly to techniques to achieve phase coherency between channels in a power combiner.
2. Description of the Related Art
Power combiners include an RF waveguide splitter that separates RF power provided at an RF input into multiple waveguide channels, solid-state amplifier chips that amplify the RF signal in each channel and an RF combiner that combines the amplified RF signals into a single amplified RF signal. The combiner may be either a waveguide combiner or a spatial combiner that utilizes free-space radiating elements. In this context, a “waveguide” is a hollow metal rectangular waveguide dimensioned for propagation of energy in a particular spectral band within the RF spectrum extending from approximately 300 MHz to approximately 1.1 THz.
To optimize combination efficiency and achieve the maximum combined power, tight phase coherency must be maintained between the channels. Each amplifier chip has a characteristic insertion phase. This phase will vary to some extent from chip-to-chip. At the higher RF frequencies in the MMW and THz regimes, fabrication tolerances in the waveguide splitter and combiner will produce phase errors that vary from channel-to-channel.
One approach to achieving phase coherency is to measure the phase of a number of amplifier chips and select chips having a similar phase within a specified tolerance. This approach is feasible if you have a sufficiently large pool of amplifier chips from which to select and if the phase errors in the waveguide splitter and combiner are negligible.
Another approach is to pair each amplifier chip with a phase-shifter chip, which can be tuned via a control signal to adjust channel phase. This approach is feasible, for example, in the X and KA bands toward the lower frequency end of the RF spectrum. At higher frequencies in the MMW and THz regimes, the phase-shifter chips become very lossy.
Another approach is to insert a wedge of dielectric material into each channel to essentially “shim” the phase. Calibration of multi-channel power combiners using this approach can be very tedious, practically impossible for more than 2 channels. The waveguides have to be disassembled, the wedge inserted and the waveguide reassembled. The phase of each channel can be measured independently to get an initial solution with different wedges being inserted until each channel has the same nominal phase. However, there is some degree of cross-coupling between the channels. Consequently, to achieve optimal performance one must calibrate for maximum power with all channels. The adjustments to achieve maximum power can be highly iterative and difficult to achieve optimal performance.